With a new book, Big Brother, the author behind We Need to Talk about Kevin tackles another hot-button issue: obesity. The ever-opinionated Lionel Shriver gives Boris Kachka an earful.

A long visit to Lionel Shriver's London townhouse is so uncomfortable it almost comes round to being cozy. "You might want to keep your coat on," the American expat novelist says, her breath fogging up the entryway. She refuses to use the gas heat ("they're price gougers") and rarely lights the woodstove before nightfall. Two hours later, she'll think to offer a cup of tea. She herself never eats until after evening calisthenics; bedtime is around 3 a.m. "I'm very routinized," Shriver explains. "I'm inflexible. Which is why we're not having lunch!" Her furrowed expression softens as she belts out a slightly smoky laugh, a relic of the one-per-day habit she recently traded in for an e-cigarette.

"Jeff gave me a really hard time about inviting you over to the house," she says later—meaning Jeff Williams, her jazz drummer husband of 10 years, whom she fictionalized in her best-selling novel The Post-Birthday World. "He said, 'He's just gonna criticize your furniture.' Which is worthy of criticism, by the way!" The living-room chairs are indeed faded and mismatched, but the place has its charms: a rough-hewn dining table, a vintage stove, and Shriver's own sculptures, which range from a figure in the fetal position to a dazed-looking woman in a bath.

Shriver's fictional characters are as odd and indelible as their creator. Most famously, there's the deeply flawed mother in her seventh published novel, We Need to Talk About Kevin, whose son ends up going on a killing spree at his high school. Published in 2003, the book became an "underground feminist hit," per one paper, then a genuine hit, and finally a literary totem of America's national insanity. In 2011, the book was turned into a haunting movie—and then, last year, there was the shooting in Newtown, Connecticut. Shriver supports gun control, but she has to keep reminding people that Kevin used a crossbow. Her novel wasn't about politics. "I was more interested in the pure malice itself," she says, "which is a larger question and more interesting and more literary—i.e., more my business."

A big part of Shriver's business, across 11 novels, is to tear down the old literary wall between "big idea" fiction and the "personal" (read: feminine) variety. "All novelists write from personal experience," she says. "Reading the newspaper is a personal experience." But her own story does course beneath the surface of her books. Raised in North Carolina by a Presbyterian minister and a homemaker mother, the teenage Shriver forswore her given name—Margaret Ann—her religion, and, eventually, the South. After graduating from Barnard in 1978, she spent 15 years in difficult foreign locales before putting down roots in an unglamorous section of South London.

Shriver's novels most often follow paths not taken, imaginary branches of her own life. Instead of having a child, she created the mother of a monster in Kevin. After deciding to leave her previous partner and marry Williams, she came out with The Post-Birthday World, which describes two alternate relationship-realities. And A Perfectly Good Family imagines what it might have been like if Shriver's parents had died, leaving the writer and her two brothers to wrangle over their inheritance.

Her still-living parents responded to that novel with long, scathing letters threatening to disinherit Shriver. Her younger brother didn't talk to her for two years. Relations have since thawed, but, she says, "I don't think we have ever properly talked it out." And while she isn't apologetic—"nothing short of a hagiography would have sufficed"—she isn't immune to regret. When her parents finally do pass on, she says she'll feel "a sense of loss contaminated with a sense of guilt and culpability and inade­quacy and just generally feeling like a shithead."

Shriver's new novel, Big Brother (June, Harper), could easily be read as a reckoning—with not just familial regret but also her youthful faith in self-reinvention. Its real subject is her own black sheep older brother, Greg, her only family member to actually love A Perfectly Good Family. Big Brother's narrator, an Iowa woman named Pandora, gets a visit from her older brother, a down-and-out New York–based jazz pianist who's recently lost all his money and gained 223 pounds. Overriding both their former estrangement and her own tidy family, Pandora takes leave of her very thin husband in order to save her brother's life.

Philip Friedman/Studio D

Along the way to a shattering conclusion, Shriver ponders our collective inability to stop obsessing about food. "You couldn't help but wonder," she writes, "what earthly good was a microprocessor, a space telescope, or a particle accelerator, when we had mislaid the most animal of masteries." Beneath the big ideas, Big Brother is also Shriver's most personal novel. "She can be merciless," says Gail Winston, Shriver's editor at HarperCollins. "But I think this shows a certain fragility and vulnerability that probably is somewhat new—mainly because of the backstory."

In November 2009, Shriver was in her London kitchen when Greg's doctor called from New York. Her brother had become morbidly obese over the past decade, and, at 55, was in and out of the hospital. He had a genius IQ, she says, and was a successful sound engineer before his life went downhill. Shriver asked the doctor if Greg could handle gastric-bypass surgery, and he said yes—but he would need someone in New York to look out for him. Shriver owns a home in Brooklyn, where she and Williams spend their summers. "I just wondered, Did I have it in me? I wasn't sure." She pauses, raising her voice. "And then two days later he was dead. I never had to decide. I was never tested. I'm not much of an altruist. I felt guilty about my sense of relief."

"I don't think you're really a misanthrope," Williams tells his wife, having joined us in the frigid living room. She agrees, to a point. "I do not take an instant dislike to people. It's a generalized despair. Anyone who has gotten nearly to the age of 57 and not at least had a moment of loathing of her own species hasn't been paying attention." (She's actually 55.) Above Shriver's kitchen table is her "masterpiece," a framed collage of newspaper clippings: Police Taught How to Climb 3ft Ladder; Gardener Drowns in a Water Barrel; One in Ten Thinks That Life is Not Worth Living. "There are many themes to it," she says. "But I guess the larger point is how dumb life is."

She also tends to yell at the TV news. Williams says the DVR has improved their experience, though pausing the program for Shriver's riffs on the topic du soir can stretch a 50-minute broadcast to an hour and a half. One recent rant came the Sunday after the Newtown massacre, during Obama's speech. "At no point," she remembers, "did he ever mention the mother," Adam Lanza's first victim. "There was in the omission the implicit accusation that she was culpable."

Shriver says she had "more or less hung up my school-shooting hat," turning down writing assignments and talking-head gigs. "For me to constantly be consulted as an expert in school shootings is ludicrous. I made one up." But Obama's omission cut to the heart of We Need to Talk About Kevin, in which the mother suffers from knee-jerk ostracism. Moved, she wrote a column for The Guardian headlined The Scapegoating of Nancy Lanza.

Though she doesn't love the label public intellectual, Shriver owes a lot of her British fame to columns and broadcast appearances, in which she serves as the designated American. "I'm definitely used as a mascot," she says. "I should have earned my way out of that category." She thinks that doing more U.S. journalism might help to raise her profile stateside, but she's hitched her wagon to England and is considering applying for UK citizenship—partly to be eligible for the prestigious Man Booker Prize. These are high-class dilemmas for a writer who published her first six novels in almost total obscurity. "By the time I wrote Kevin," she says, "it was make-or-break. If I did not publish that book, I couldn't keep writing books." She submitted Kevin just after September 11, an inauspicious time to pitch violent stories. Her American literary agent rejected it on the basis that it was unquestionably too dark. (Shriver got revenge, of sorts, when she married Williams, that agent's ex-husband.)

Eight months later, Shriver found a new agent, Kim Witherspoon, who wanted to represent the novel. "There was such tremendous clarity and honesty about the thoughts that many women have as they're anticipating motherhood," says Witherspoon, who was pregnant at the time. Kevin eventually won Britain's Orange Prize and has now sold well over a million copies in English. Her subsequent novel, The Post-Birthday World, also hit the best-seller list.

"I think the late bloomer is much more fortunate than the early bloomer," says Shriver. "I'd definitely choose my story over Jay McInerney's…. I always felt sorry for those people. Okay, also envious: Give me that kind of break, and I'll do something with it! I wouldn't be scared of it, because I'd keep having ideas."

Shriver's books live or die by the currency of those ideas. The Post-Birthday World, consumed with women's romantic choices, was a book-club shoo-in, especially on the heels of Kevin. But her next novel, So Much for That—a critique of the broken American health-care system—was a tougher sell. "It swept the Oscars, in terms of the reviews," Shriver says. But "people don't want to buy a book about illness." Even the passage of Obamacare the month it was published didn't help. "Illness," she concludes, "reminds people of the fact that they're going to die."

Shriver places some of the blame on her bind as a woman writer. A couple of Britain's major booksellers insisted on a softer cover for So Much for That. ("It's an outrage—it has a lily on it. A fucking flower! It's a very hard-ass book…. I treasure the male readership my work has garnered, and I think it's been artificially suppressed.") One cover she does approve of is for the novel that came next, The New Republic: a picture of a cigarette in an ashtray.

In fact, Shriver wrote The New Republic in the late '90s but failed to find a publisher for it at the time. So she decided to use her newfound clout to publish it last year. It's a high-flown farce about terrorism, of all things, drawing from Shriver's years in Troubles-era Belfast. Before September 11, its problem might have been that no one took the subject very seriously. But in 2012 it faced a new obstacle: Terrorism was taken, Shriver thinks, too seriously. The New York Times' Michiko Kakutani, generally a Shriver fan, called The New Republic "cringe-making" for its "willfully breezy depiction of terrorism"—in short, "a ghastly novel."

"I had to pay a price for it," Shriver admits, though not regretfully. "I am pleased that it did seem to find some people who got it, who responded to it, who understood it." Winston, Shriver's editor, laughs uneasily when that novel comes up: "I think I'll pass on that one." The decision to publish it, with an awkward author's note, was not hers. But Winston doesn't believe it hurt Shriver's career: "No one will ignore her ever again."

Big Brother displays a subtle sensitivity to a broader readership. At Winston's suggestion, Shriver went light on the sorts of gruesome medical descriptions that may have held back So Much for That. While she insists that her writing hasn't changed a bit, her attitude certainly has. "When I was younger, what I wanted was a pat on the head," she says. "Now, if you gave me a choice—the critics will hammer you but it will be a best-seller; or you can have all the plaudits in the world, and nobody will buy it—I'd rather have the sales."

Shriver wrangled with Winston about the more big-idea-focused sections of Big Brother, which she calls "mini essays." "I call it ranting," says Winston, "and these rants are the subject of our push-pull." But Winston usually relents, recognizing the gift of a noisy novelist: "I'm of the less-is-more school, but she knows how to turn up the volume. Not everyone knows how to turn it up. Some publishers say of a book, 'Well, it's a quiet book.' That's not her problem at all."

A few hours after our first meeting, we reconvene for a late dinner in the quiet Giaconda Dining Room, an old continental place just modern enough for its Soho environs. A publisher took Shriver here once, and she liked it enough to overcome her indifference to dining out.

"My general experience of restaurants is disappointment or apathy," she says. "If I couldn't have made it myself, then I could also probably live without it." Willliams, when asked what might surprise people about Shriver, tells me, "She's one of the sweetest people I've ever known." His evidence is her generous cooking, including an elaborate sushi meal she made him before they began dating. Shriver has one fatal flaw as a cook—too much spice for most palates. "Subtlety," she says, "is not my forte." Which is another problem with restaurant food: "It's awfully bland."

Shriver bikes everywhere, no matter the weather, and her fancy Trek bike—a rare indulgence—is locked up in front of the restaurant. In the three hours between interviews, she's made an apple raspberry crumble, done her calisthenics, and changed right back into the same clothing: a black mock turtleneck, stretchy black pants, and the plainest of white sneakers. "Every once in a while," she says, "I decide to look good, and when I decide to look good, I look fantastic. And the rest of the time, I don't give a shit."

Shriver's mind is already on her next novel—or two. She's been dissuaded, for now, from writing about immigration, on which she has some impolitic opinions. "If it's just a matter of there being an Indian restaurant down the block, that's all to the good," she says. "If your entire neighborhood is full of people who don't speak your language anymore…you feel taken over." She adds, quickly, "I'm an immigrant too. And one of the reasons I find this issue interesting is exactly those kinds of contradictions." She is hesitant to discuss her in-progress novel—but she can't help herself. It's a dystopian family drama set in a near-future United States, following the collapse of the dollar. The deficit, as it happens, is one of those topics about which she can speak knowledgeably, loudly, and at great length. A proud libertarian, she finds Ron Paul's platform "very attractive," despite its flawed messenger. By the time our appetizers arrive, she's on to a full-blown mini essay. ("What else are we gonna call it? It is corrupt! Congress is corrupt!")

Maybe it's the bottle of red wine, but when the topic returns to Big Brother, Shriver becomes overwhelmed with compassion for everyone who suffers from obesity—their failures, uniquely, out there for all to see. "I'm also sympathetic with people who just weigh a little too much and then ruin their lives over it." Shriver controls her weight with her usual zeal. Only once, she says, did she let herself go. At age 17, she traveled through Britain with a friend, and "I must have gained 10 pounds. It was horrifying. So that was my walk on the wild side."

The ultimate source of Shriver's empathy—for her real brother, for the people in Big Brother, and for all of her characters, even the homicidal ones—is her instinctive disdain for moderation. "I'm a naturally very greedy person," she says, "so I am sympathetic with other people who are greedy. It has a bad reputation, the whole business of having appetite. I like having appetite." The most important "big idea" in Big Brother appears toward the end of the book, and reads, simply, "We are meant to be hungry."

"I think that's probably the best line of the book," Shriver says. "I want to be hungry. My version of total misery is eating three meals a day and having a biscuit in the afternoon. You're never hungry, and you always feel slightly sick—or I do. And you never feel you really deserve it."

While she talks, Shriver decides that her mixed fish grill is underseasoned. Pulling from her bag a small plastic canister of Turkish hot-pepper flakes, she buries the dish in a half-inch-thick layer of the stuff. The brill, sole, and scallops on her plate have gone from a delicate cream color to a garish red. "I can't tell you," she says with her mouth full, "how much it picks up the dish."

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